It wasn't accurate at all. Numerous universities tried to replicate his findings and none of them could. Other individuals have adopted the exact same diet he had for 30 days as a test and found absolutely no change to their health or wellbeing. Every time he was asked to provide data about his diet and outcomes he refused. And it wasn't until years later that he revealed he had actually been a severe alcoholic for decades.
I'm not trying to speak ill of the dead, but the guy was very much a liar by omission. He made a documentary about how unhealthy fast food is while he was pounding back booze by the truckload and the vast majority of his health issues were because of liquor, not eating big macs for a month.